[Elecraft] K3: no output on "line out"?

[Franki ON5ZO]

I am trying to get set up for RTTY with an MK2R+ and N1MMLogger / MMTTY.
It seems the line out is dead: I don't get a waterfall on the screen. When I 
plug the cable from the other radio in the K3's slot on the MK2R+, I get a 
signal. I tested the cable between rig and interface which seems OK.
So my conclusion is: nothing comes out of the LINE OUT. I looked through the 
manual but only found the LIN OUT level parameter which I cranked up to the 
max but that doesn't fix it.

Any ideas? Am I forgetting something?
- Franki ON5ZO